Processes for the vapor phase oxidation of diolefins to furan utilizing selected catalysts such as heterogeneous bismuth molybdates, manganese molybdates and tungstates and processes for the oxidative dehydrogenation of alkenes or alkadienes to furan with selected transition metal phosphate catalysts are well known in the art. Although such processes use low to high concentrations of hydrocarbon in the gaseous feed mixtures, generally, the hydrocarbon feed level is no greater than 5%, in which case the concentration of furan in the resultant off-gases is less than 1%. Because of the low concentrations of furan realized by such processes, isolation of the furan may be excessively costly and/or difficult due to process inefficiencies, such as inordinately high compressor demands, vapor entrainment and the like.
Since the furan concentration index, a measure of the effectiveness of the process in producing furan, is the product of the concentration of C.sub.4 -hydrocarbon in the feed mixture.times.C.sub.4 -hydrocarbon conversion.times.furan selectivity, it is imperative that these three variables be maximized for high furan production. The term "C.sub.4 -hydrocarbon conversion" is defined, in %, as 100 times the number of moles of C.sub.4 -hydrocarbon converted to oxidation products other than butadiene divided by the number of moles of C.sub.4 -hydrocarbon in the initial feed mixture. The term "selectivity" is defined, in %, as 100 times the number of moles of a specific oxidation product produced in the reaction and normalized to a C.sub.4 -base divided by the total number of moles of C.sub.4 -hydrocarbon converted to oxidation products other than butadiene.
One method of obtaining a higher ultimate furan concentration is to increase the hydrocarbon feed concentration without incurring simultaneous decreases in hydrocarbon conversion and/or furan selectivity. In general, such an approach, using known bismuth molybdate catalysts and high concentrations of the unsaturated hydrocarbon, at conversion levels of 12% or greater, even in the presence of excess oxygen in the gas exit stream, results in marked decreases in furan selectivity and sharp increases in the production of carbon oxides and, ultimately, cracking products due to severe coking of the catalysts.
The primary object of this invention, therefore, is to provide a catalytic process for the vapor phase oxidation of a diolefin to furan, which process uses high concentrations (at least 10%) of the hydrocarbon in the feed stream, without significant losses in hydrocarbon conversion or furan selectivity, and yields a furan concentration in the off-gases of at least 1%.
All percentages disclosed herein, unless otherwise specified, are mole percentages.